The long-range goal of our research is to understand the mechanism for co-receptor selectivity by HIV-1 (binding to CCR5 and CXR4 receptors) and the role of the membrane proximal domain of the envelope glycoprotein gp41 in HIV-1 fusion. The binding of HIV-1 and its fusion to target cells is mediated by the gp120 and gp41 envelope glycoproteins of the virus. The third variable loop (V3) of gp120 is a major neutralizing determinant of HIV-I. Segments of V3 form the binding site for the gp120 co-receptors on T-cells and macrophages and its sequence determines the virus phenotype, i.e. whether it binds the CCR5 chemokine receptor and infects macrophages (designated R5 virus), or it binds CXCR4 and infects T-cells (designated X4 virus). Deletion of V3 or its binding to antibodies prevents HIV-1 fusion with its target cells, thus abolishing its infectivity. We suggest that the selectivity of HIV-1 is determined to a major extent by alternative conformations of V3. The binding of gp120 to the chemokine receptors induces conformational changes in gp41 that mediate viral fusion with the target cell. The tryptophan-rich C-terminal membrane proximal domain of gp41 contains the only neutralizing epitope within gp41, and partially overlaps with the sequence of the peptide DP178, a strong entry-inhibitor of HIV-1 under clinical trials. Given the important biological properties and lack of complete structural information on gp120 and gp41, multi-dimensional NMR techniques will be used to decipher the missing structures of key components of these envelope proteins, as free peptides, constrained peptide analogs, when bound to broadly neutralizing anti-HIV-1 antibodies, and in the context of the gp41 protein. The structure of the alternative conformation of V3 recognized by HIV-1 neutralizing antibodies will be determined. Constrained peptides mimicking these conformations will be synthesized and their structure determined. The characterization of CCR5 and CXCR4 selectivity to the constrained conformations will have profound implications on our understanding of the mechanism for co-receptor selectivity and HIV-1 neutralization by antibodies directed against the V3 loop. The structure of the N- and C-terminal regions of gp41 extra-cellular domain will shed light on their involvement in membrane fusion, and the importance of the C-terminal half of DP178 for antiviral activity and for eliciting neutralizing antibodies. This study will provide invaluable information for constructing immunogens for HIV-1 vaccines and for developing anti-HIV-1 therapeutics.